In oil wells, natural gas wells, and the like (hereinafter also collectively referred to as “oil wells”), oil country tubular goods are used for extraction of underground resources (e.g., petroleum, natural gas, etc.). Oil country tubular goods, which are steel pipes, are configured to be sequentially connected to each other, and threaded joints are used for the connection.
Such threaded joints are generally classified into two types, a coupling-type joint and an integral-type joint. A coupling-type threaded joint is constituted by a pair of tubular goods that are to be connected to each other, of which one is a steel pipe having a longer length and the other is a coupling having a shorter length. In this case, the steel pipe is provided with a male thread formed on the outer periphery at each end portion thereof, and the coupling is provided with a female thread formed on the inner periphery at each end portion thereof. The male thread of the steel pipe is screwed into the female thread of the coupling, thereby making up a joint between them. An integral-type threaded joint is constituted by a pair of steel pipes as tubular goods that are to be connected to each other, without a separate coupling being used. In this case, each steel pipe is provided with a male thread formed on the outer periphery at one of its opposite end portions and a female thread formed on the inner periphery at the other thereof. The male thread of one of the steel pipes is screwed into the female thread of the other of the steel pipes, thereby making up a joint between them.
In recent years, from the standpoint of improving the manufacturability of oil country tubular goods, there is an increasing need for using a threaded joint of the integral type. This is because no separate coupling is required.
When making up steel pipes, lubricating grease (dope) is applied to the male thread and the female thread. The purpose of this is to prevent galling in the threads and also to enhance the sealing performance of the threaded joint. Conventionally, as the lubricating grease, lubricants specified by API (American Petroleum Institute) standards (hereinafter also referred to as “API dope”) are widely used. API dope contains heavy metals such as Pb (lead) and exhibits high lubricity.
In recent years, environmental regulations have become more stringent. Thus, the use of API dope has been restricted, and a need has arisen for use of lubricating grease free of heavy metals (hereinafter also referred to as “green dope”). However, green dope has lower lubricity than API dope. Because of this, in the case of using green dope, it is necessary to apply an electroplated coating such as a copper coating to the surface of at least one of the male thread and the female thread. The purpose of this is to prevent galling in the threads by compensating for the insufficient lubricity.
When applying an electroplated coating to a coupling-type threaded joint, the coating is applied to the female thread of the coupling. Threaded joints having an electroplated coating on the female thread of the coupling exhibit high reliability. Because of the high reliability, when applying an electroplated coating to an integral-type threaded joint, too, it is increasingly desired that the coating be applied to its female thread on the pipe end portion of the steel pipe.
Japanese Patent Publication No. S63-6637 (Patent Literature 1) discloses an apparatus for applying an electroplated coating to a male threaded region formed on one of the pipe end portions of a steel pipe, i.e., to the outer peripheral surface at a pipe end portion of a steel pipe.